The invention relates to combustion heating. More particularly, the invention relates to pulse combustion heating apparatus and methods wherein primary and main burners are arranged in fluid communication to provide a combustion system having pulse operating characteristics derived from the combination of the two burners.
In the pulse combustion burners of the Helmholtz type, an oscillating or pulsed flow of combustion gases through the burner is maintained at a frequency determined by burner component geometry and fuel supply characteristics, including the mixing of components thereof. Typically, a combustion chamber of a given size cooperates with a tailpipe or exhaust pipe of specific dimensions to provide explosive combustion cycles, thermal expansion of the combustion gases, and oscillating gas pressures which provide the pulsed flow of combustion gases through the burner. In order to make the pulse combustion process self-sustaining, the oscillating gas pressures may be used to provide self-feeding of a combustible gaseous mixture which generally comprises air and a gaseous fuel such as natural gas.
The operation and stability of pulse combustion burners are dependent upon the burner geometry and the degree of air and fuel mixing as indicated. Also, the ease of initiating ignition and maintaining stable operation are affected by these factors. Accordingly, pulse combustion burners are not readily amenable to operating over a wide turndown ratio. The turndown ratio in a typical pulse combustion burner is in the range of 15% to 35% of its designed fuel energy input rate. If the input rate is reduced below a minimum operating value, the process stability self-decays as reduced operating pressures result in correspondingly reduced fuel input rates until burner shutdown occurs. In a somewhat related manner, air and/or fuel supply variations may cause significant changes in the operation of the burner, including burner shutdown.
The close dependency between pulse combustion operation and burner geometry also makes scaling difficult. Presently, scaling is substantially a trial and error process based in part upon empirically developed relationships and data developed in respect to the particular scaling application. Scaling is increasingly more difficult as the absolute value of the fuel input rate increases. Thus, it is significantly more difficult to scale-up by a factor of five a 1,000,000 BTU/hr. burner as compared with a 100,000 BTU/hr. burner.
U.S. Pat. No. 3,194,255 to Marchal et al. discloses a system wherein a resonant burner exhausts into a non-resonant burner to cause periodic combustion of gases in the non-resonant burner. The exhaust gases of the non-resonant burner are directed into an optional final or tail burner. Another mixed burner system is disclosed in U.S. Pat. No. 4,473,348 to Tikhonovich et al. In this patent, a continuous auxiliary burner exhausts its combustion products into a main burner between the feed pulses to smooth out the combustion in the main burner.